Roast the poblano over an open flame or 4 inches below a broiler, turning regularly until blistered and blackened all over, about 5 minutes for an open flame, about 10 minutes for a broiler. Cover with a kitchen towel. Let them cool until you can handle them. Rub the blackened skin off the chiles and pull out the stem and seed pod. Cut into 1/2-inch pieces.
Spread the onion over the bottom of a slow-cooker. Sprinkle with salt. Add half the pork, potatoes, tomatillos, and poblanos, salting each of the layers. Sprinkle on all the dried porcini mushrooms and half of the epazote (cilantro) leaves. Add the second half of the ingredients, sprinkling salt evenly over each layer. (This is the only salt in the dish, so make sure to add enough.) Cover and slow-cook on high for 6 hours (the dish can hold on a slow-cooker’s “keep warm” function for several hours.
Taste and season with more salt, if needed, then ladle your pork stew into warm bowls and enjoy.